It is well known that, as the rotational velocity of a shaft increases, the shaft passes through several progressively higher speed ranges known as critical speeds. If a shaft is unsupported intermediate its ends and is rotated at or near what is known as its first order critical speed, centrifugal forces periodically exceed centripetal forces at the shaft's median portion to bow the shaft outwardly from its normal rotational axis, whereupon shaft rotation becomes unstable and excessive vibration occurs. Such vibration may quickly damage bearings and bearing support structures, as well as the shaft, and may result in impact of the shaft against housing elements.
After a rotating shaft reaches a soeed greater than its first order critical speed, vibration subsides and the shaft rotates smoothly. Since the second order critical speed of any shaft is considerably greater than its first order critical speed, there is a wide range of speeds between the first and second order critical speeds at which the shaft will operate with little or no vibration and with little effect on the bearings, other than normal wear.
While a shaft may be operated safely at speeds below its first order critical speed, the preferred speed for a particular operation may be above the first critical speed. In such a case, in order to avoid undesirable shaft vibration and possible impact with the shaft housing during acceleration or deceleration through the first order critical speed, it is desirable to increase the shaft's first order critical speed to a speed above the maximum desired operational speed by altering the shaft's mode of vibration.
A shaft's first order critical speed is inversely related to its length and directly related to its rigidity which, in turn, is directly related to the shaft's diameter. Attempts to increase the first order critical speed of a shaft have, through changes in shaft design, encountered various design problems such as increased bearing size and overall weight.
Prior attempts to damp shaft vibration or to otherwise modify shaft vibration modes have had only limited success. Prior approaches to the problem are described in Matheny, Jr. U.S. Pat. No. 3,897,984, issued Aug. 5, 1975, and Seibel 2,652,700, issued Sept. 22, 1953.
Matheny, Jr. provides a shaft support comprising a generally annular resilient member disposed slightly eccentrically about the shaft generally centrally thereof. The resilient member has means associated therewith for exerting a radial preload force on the shaft. The force exerting means includes an annular roller bearing disposed about the shaft and contacting a sleeve thereon. Shaft vibration is thereby damped but at the expense of constant bearing contact and, therefore, constant wear of the shaft and the bearing structure, with consequent power losses due to friction.
In Seibel, a shaft extends through the central aperture of a damper plate which is mounted to the shaft housing by springs. The plate is contacted by a sleeve on the shaft during rotation thereof to absorb energy to prevent transmittal of shock to the supporting structure. Structures such as in Seibel tend to be noisy, involve relatively many parts, and suffer excessive wear.